ADSL is a newly standardized transmission technology facilitating simultaneous use of normal telephone services, data transmission of 6 Mbit/s in the downstream and Basic-rate Access (BRA).
ADSL can be seen as a FDM system in which the available bandwidth of a single copper-loop is divided into three parts. See figure . The baseband occupied by POTS is split from the data channels by using a method which guarantees POTS services in the case of ADSL-system failure (eg. passive filters).
Figure: Frequency Spectrum of ADSL